Spread spectrum clocking is now widely used in the form introduced by U.S. Pat. No. 5,488,627, assigned to the assignee to which this invention is assigned. U.S. Pat. No. 5,631,920, also assigned to the assignee to which this invention is assigned, is directed to digital implementation and control of such circuits. U.S. Pat. No. 6,404,834, also assigned to the assignee to which this invention is assigned, is to spread spectrum clock signals which are segmented by at least 10 dB reductions so as to meet certain proposed CISPR-22 rules.
As processor clock speeds continually increase, the need for methods to mitigate electromagnetic interference (EMI) become more important. Previous methods have been designed for clocks operating below 1 gigahertz (GHz) where the primary emissions are also below 1 GHz. At the current frequencies a bandwidth of 120 kHz was considered indicative of EMI level and measurement was made at that bandwidth.
At frequencies above 1 GHz a bandwidth of 1 megahertz (MHz) is currently considered indicative of EMI, and measurement at 1 MHz bandwidth is required by the Federal Communication Commission (FCC). CISPR, the European regulatory commission is proposing similar regulations. Current implementation of spread spectrum clocking provides little EMI reduction for clock harmonics above 1 GHz.
Accordingly, modifying current aspects of spread spectrum clock generation to accommodate the higher frequencies is needed. Modification which accommodates both the higher frequencies and lower frequencies from the same clock source is required, even with some compromise from the ideal for higher and lower frequencies.
The output of the spread spectrum clock generator is determined by the waveform used to angle modulate a clock circuit. One skilled in the art would recognize that angular modulation could be described in terms of either phase modulation (PM) or frequency modulation (FM). Also, a phase shifter circuit or a frequency modulator circuit may be used. The derivative of a phase modulation waveform will result in a frequency modulation waveform. When a phase modulation waveform is applied to a phase shifter circuit and a frequency modulation waveform, which is the derivative of the phase modulation waveform, is applied to a frequency modulation circuit, both circuits will have identical outputs.
With the introduction of new modulation characteristics, it is required to make modifications to the methods and circuits that provide the spread spectrum clock generation output. Various modifications and improvements are also required to some cited patents to be able to perform the desired function with a minimal number of circuits.